Innovations in Corrosion and Materials Science (Discontinued) - Volume 6, Issue 2, 2016

Very high ultimate autogenous shrinkage of ultra-high-performance fiberreinforced concrete (UHPFRC) is normally obtained due to the use of low water-tobinder ratio (W/B) and admixtures with fine particle sizes, such as silica fume and filler. In particular, a large part of the shrinkage is generated at a very early stage, and this leads to a high potential of early-age cracking in thin plate structures made by UHPFRC due to the restraint of shrinkage. For this reason, several studies have been performed with regard to the shrinkage behaviors of UHPFRC under both free and restrained conditions. In this study, the previous studies regarding the shrinkage characteristics of UHPFRC are extensively reviewed. In addition, several important concerns, which have not been clearly reported, with regard to the free and restrained shrinkage properties of UHPFRC are addressed and discussed.

Concrete’s durability is the ability to resist deterioration over time, and maintain its engineered properties like strength, form and aesthetics. Durable concrete is more sustainable and more economical, and a variety of techniques can be used on hardened concrete to evaluate its durability. The techniques outlined are focused on: freeze and thaw and scaling, alkali-silicate reactivity, petrographic examination, abrasion and erosion, chloride content determination, steel reinforcement corrosion potential, RCP, and fluid penetration resistance.

Background: Concrete structures can be damaged by many chemical or physical processes. Where such damages affect appearance, serviceability or structural integrity, repair may be required. 10 decommissioned concrete ships form a massive floating breakwater on the Malaspina Strait in the City of Powell River in British Columbia, Canada. The ships are subjected to extremely onerous conditions, namely in the salt water splash zone under seasonal freeze-thaw cycling. This paper provides anecdotal evidence based on condition assessments after repairs were carried out, on how a well-matched repair concrete can significantly extend the service life of these and similar sorts of marine structures. Methods: Condition assessment was carried out using sounding, visual examination, photo and video documentation, compressive strength testing, splitting tensile strength testing, chloride ion concentration testing, and half-cell potential testing. Results: It was found that the areas which had been repaired had fared very well, with no observable deterioration. The non-repaired areas of the structures had continued to deteriorate, particularly in nonsubmerged areas, close to the waterline, along with new impact damage. Conclusion: Durable concrete repairs can be achieved even for concrete exposed to extreme environmental conditions. Key conditions for such repairs are: complete removal of the deteriorated portions of the original concrete, removal of all loose corrosion products from exposed reinforcing steel, preparation of a clean, firm, rough, surface dry but partially or fully water saturated, substrate surface, selection of a compatible repair material, use of an application technique which facilitates high bond strength between substrate and repair material, adequate curing, and protection of the repair material while immature.

Background: Numerical studies on the shear behavior of reinforced concrete (RC) structures repaired with fibre-reinforced polymer (FRP) sheets are relatively limited because of the complexity of the shear mechanisms. Almost all of the studies are conducted at the component-level due to the computational time and memory storage limitations associated with detailed finite element (FE) tools. Methods: A multi-scale analysis framework was recently developed based on the substructuring technique for the nonlinear analysis of RC structures. In this study, the proposed framework was employed for modelling and analysis of structures strengthened with FRP. In this procedure, the repaired components were modelled in finer detail using a 2D FE program, while the rest of the structure was modelled with a computationally-fast frame analysis program. The sub-models were connected using a newly developed interface element, the F2M element, which satisfies equilibrium and compatibility conditions and provides an accurate shear stress distribution for cracked concrete at the interface. A practical and reliable method was presented to model FRP-related mechanisms for repaired components. Link elements were used to consider bond-slip effects and peeling-off phenomena of FRP sheets. The confinement enhancement of FRP was modelled by addition of an out-of-plane smeared component to the corresponding rectangular RC elements. Second-order material effects such as tension stiffening and crack spacing were taken into account using proper models. Results and Conclusion: The application of the proposed modelling procedure was investigated by analyzing a RC frame with shear-critical beams repaired with FRP sheets. The analysis found that insufficient consideration of shear-related effects can lead to significant overestimations of strength and deformation capacity, and inaccurate predictions of structure behavior. Most frame analysis procedures, including plastic hinge and layered analysis approaches, require difficult assumptions and inputs to account for shear mechanisms which can significantly affect structural response. In general, the mixedtype analysis was able to accurately predict the behavior of the structure particularly in terms of stiffness, peak load, ductility, failure mode, and energy dissipation. The proposed method was capable of considering the effects of previous damage with the use of stress and strain history of the elements. In addition, the change in the damage mode prior and after the repair of the frame structure was captured accurately.

Background: In comparison with the prediction of bending failure, the existing design models for the calculation of the shear resistance are still rather vague. Therefore, currently strong efforts are being undertaken to improve and update current design models. In order to better understand the mechanism of shear failure and finally to identify the degree of shear degradation in existing structures in use, refined measuring techniques have to be applied for monitoring the short-term and long-term loadbearing behavior. Objective: A Monitoring system has to provide reliable information on the shear degradation development. The evaluation and optimisation of standard monitoring systems regarding shear failure based on shear testing are presented in this paper. Method: Different conventional structural health monitoring methods were used to monitor the external and internal deformations caused by shear loading, in order to evaluate the sensitivity and correlation of the different measurements. Based on adapted material models, as well as a nonlinear numerical finite element simulation together with a sophisticated monitoring layout, the internal flow of forces and the crack formation process can be recorded and visualized. The comparison of the experimentally required data with the provided numerical solutions allows an optimization of the monitoring system. Results and Conclusion: The sensitivity studies performed so far show that different measurement devices exhibit a characteristic sensitivity to the behavior of the structure at each testing stage. Evaluating the data of sensors which are sensitive enough regarding shear failure enables the approximation of the shear failure initiation point at an early stage.

Background: During the production of engineering metals there is always a consolidation stage at which smaller particles of metal are consolidated to create macroscopic pieces for engineering applications. Powder metallurgy is exemplary. Such processes involve the impingement of oxides, creating unbonded double films, called bifilms, acting as cracks. Such consolidation cracks appear to be ubiquitous throughout metallurgy and engineering. They appear to be the Griffith cracks required for failure by cracking, being responsible for initiating failure by cracking. Interestingly, they appear also to be necessary for invasive corrosion processes such as pitting, filiform corrosion, and possibly stress corrosion cracking and hydrogen embrittlement. Methods: Possible alternative sources of Griffith cracks have been researched. These include nucleation of pores and/or cracks in the liquid, solidification defects, and lattice defects in the solid, such as vacancy condensates and dislocation pile ups. Results: No alternative sources of Griffith cracks have been discovered. The only defect capable of the initiation of cracks or invasive corrosion appears to be the bifilm. Conclusions: Although consolidation by the casting of liquid metal into ingots and other shapes has traditionally been carried out poorly, resulting in dense bifilm populations, explaining the unreliability of traditional cast materials, the casting route is now capable of consolidating engineering products such as ingots and shaped castings with unequalled perfection. In principle, for the first time, it seems that the metals incapable of failure by either cracking or invasive corrosion are now possible.

Background: Potentiodynamic polarization experiments were performed on Alloys IN601 and C22 to evaluate their susceptibility to localized corrosion in hydrochloric acid (HCl) at pH2. The tested specimens were evaluated by auger electron spectroscopy (AES) and scanning microscopy (SEM). Nickel chromium alloys consist of surface oxide layers and corrosion products with metallic materials beneath. Methods: Electrochemical techniques are used to examine the pitting tendencies of specimens of IN601 and C22 immersed in a concentrated hydrochloric acid solution (pH2). Techniques include potentiodynamic anodic polarization to determine the active-passive characteristics of alloys and scanning, X-ray, and Auger electron spectroscopy to characterize specimen surfaces in terms of pitting morphologies and for surface analysis of oxide layers. Results: This paper examines the effect of HCl solution on the pitting behaviour of these alloys. Results show that IN601 alloy was characterized by more pitting than C22. Conclusion: There are deeper oxide layers and corrosion products formed on sample surfaces of alloys C22 than in IN601. Furthermore, C22 exhibits a smaller hysteresis loop than IN601, thereby indicating that it has no pitting corrosion in concentrated HCl than IN601.

Background: AZ91D is one of the most widely used die-casting magnesium alloys. The corrosion processes of these materials is complex due to the simultaneous action of metallurgical and surfaces states. Mapping the corrosion behavior in chloride-containing aqueous solutions is of practical interest to guide the safe use of magnesium alloys against deterioration and accompany the evolution of the electrochemical response with time. Methods: The corrosion behavior was assessed using electrochemical tests consisting of potentiostatic polarization and potentiodynamic polarization for the AZ91D alloy after immersion for up to 7 days in NaCl solutions with different chloride concentrations. Results: A corrosion map was elaborated using the information obtained from the electrochemical tests. Immunity, passivation and corrosion zones were identified. The chloride concentration strongly affects the passivation range. Conclusion: The corrosion region becomes wider as the chloride concentration increases at a given applied potential. The surface oxide film is, therefore, protective in the most diluted solutions (0.01 M and 0.03 M) below -1.20 V and becomes more prone to pitting corrosion for more concentrated solutions. The corrosion map evolved from 1 to 7 days of immersion, suggesting that transition between the different zones can be affected by the exposure conditions.